The present invention relates to a process for the preparation of particularly reactive and low-viscosity reaction products of polyisocyanates which contain activated ethylenically unsaturated groups which react by polymerization under the action of actinic radiation, and the use of these reaction products in coating compositions, and particularly scratch-resistant coatings obtainable therefrom.
In this connection, the term “reactive” relates to the reactivity with respect to curing by means of actinic radiation, i.e. to the tendency to crosslink under the influence of radiation.
Curing of coating systems carrying activated double bond by actinic radiation, such as e.g. UV light, IR radiation or also electron radiation, is known and is established in industry. It is one of the fastest methods of curing in coating technology. Coating compositions based on this principle are therefore called radiation- or actinic-curing or -curable systems.
Due to the ecological and economic requirements of modern lacquer systems of using as little organic solvent as possible or even no organic solvent to adjust the viscosity, there is the desire to use lacquer raw materials which are already low-viscosity. Polyisocyanates having an allophanate structure, such as are described inter alia in EP-A 0 682 012, have been known for this for a long time.
In industry, these are prepared by reaction of a mono- or polyhydric alcohol with large amounts of excess aliphatic and/or cycloaliphatic diisocyanate (cf. GB-A 994 890, EP-A 0 000 194 or EP-A 0 712 840). Unreacted diisocyanate is then removed by means of distilling off in vacuo. According to DE-A 198 60 041, this procedure can also be carried out with OH-functional compounds having activated double bonds, such as e.g. hydroxyalkyl acrylates, but difficulties occur in the preparation of products of particularly low monomer content. Since the distillation step must proceed at temperatures of up to 135° C. in order for the residual isocyanate content to be lowered sufficiently (<0.5 wt. % of residual monomer), during purification double bonds can already react by polymerization under thermal initiation, so that flawless products can no longer be obtained.
The preparation of allophanate-containing, radiation-curing polyurethane-based binders of low monomer content is described in EP-A 0 867 457 and U.S. Pat. No. 573,925. Nevertheless, these binders carry no activated double bonds, but unreactive allyl ether groups (structure R—O—CH2—CH═CH2). The addition of reactive diluents (low molecular weight esters of acrylic acid) which introduce the necessary UV reactivity is therefore required.
There has likewise been no lack of attempts to prepare allophanates indirectly from isocyanate derivates other than urethanes and isocyanates. EP-A 0 825 211 thus describes a process for building up allophanate structures from oxadiazinetriones, although no radiation-curing derivatives with activated double bonds are mentioned here. Application to the particular circumstances of radiation-curing systems is described in WO 2004/033522.
Another route is the opening of uretdiones (cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419 and US-A 2003 0153713) to give allophanate structures, which it has likewise already been possible to apply successfully to radiation-curing systems (WO 2005/092942).
Both routes require refined raw materials as the starting material, and lead only to an allophanate product rich in by-products.
U.S. Pat. No. 5,777,024 describes the preparation of low-viscosity radiation-curing allophanates by a reaction of hydroxy-functional monomers which carry activated double bonds with isocyanate groups of allophanate-modified isocyanurate polyisocyanates. The radicals bonded via the allophanate groups are saturated here, as a result of which a possible higher functionality is eliminated.
EP-B 694 531 describes a multi-stage process for the preparation of hydrophilized allophanates having radiation-curing groups. In this, however, an NCO- and acrylate-functional urethane is first prepared, which is hydrophilized and, after addition of a further NCO- and acrylate-functional urethane, is then allophanated. Temperatures of 100-110° C. are stated as the process temperature for the allophanatization.
Finally, EP-A 1 645 582 described a process which leads to low-viscosity allophanates starting from simple diisocyanates by reaction with hydroxy-functional acrylates without distillation of the products. Nevertheless, a disadvantage of this process is that a satisfactory rate of reaction is to be achieved only with ammonium salts which are difficult to access. The viscosities of the products described are also not as low as e.g. the viscosities of the allophanates which are obtainable by the process described in EP-A 0 825 211. The reactivity furthermore is also in need of improvement.
Lower viscosities can be achieved in the method described in EP-A 1 645 582 by using a suitable basic zinc catalysis such as is described in EP-A 2 031 005. A further improvement in the viscosity can also be achieved if suitable blends of various hydroxyacrylates are used, as EP-A 2 031 003 teaches. Above all, the combination of the two processes leads to products with very low viscosities. Nevertheless, above all the scratch resistance of coating based on such binders is in need of improvement. The reactivity also cannot be increased in this way.
The object of the present invention was therefore to provide a process with which very low-viscosity allophanates which can be crosslinked by actinic radiation (radiation-curing allophanates) and have an increased reactivity can be provided, and to provide coating compositions based on these which lead to coatings having a particularly high scratch resistance.